1. Field of the Invention
This invention relates generally to a reactive material suitable for use as a shape-charge liner in a penetrating warhead and in reactive fragments in a fragmentary warhead. More specifically, the invention relates to a reactive material comprising a fluoropolymer and a metal filler. This reactive material is capable of being safely processed at temperatures significantly below the thermal autoignition point of the reactive material.
2. State of the Art
A penetrating warhead 2 used in a projectile or missile typically comprises a case 4, an explosive material 6, an initiator, and a liner 8, as shown in FIG. 1A. The case 4 is generally a cylindrical tube comprised of steel, plastic, or a composite material. At least a portion of the case 4 is typically filled with the explosive material 6. When the explosive material 6 in the warhead 2 is detonated, the liner 8 forms a high-velocity jet that has a high kinetic energy capable of penetrating solid objects, such as a target. The liner 8 is formed from a solid material that is formed into a jet responsive to detonation of the explosive charge. The liner material is typically a high density, ductile material, such as a metal, a metal alloy, a ceramic, or a glass. The metals commonly used in liners include copper, aluminum, depleted uranium, tungsten, or tantalum. In addition to penetrating warheads 2, fragmentary warheads 10 are commonly used. As illustrated in FIG. 1B, the fragmentary warhead 10 typically comprises fragments 12 of material that are projected at a target upon detonation of the explosive material 6 of the warhead 10. The fragments 12 must be able to withstand the explosive force of the detonation, otherwise the force commonly breaks up the fragments, thereby reducing their ability to penetrate the target.
Depending on the mechanical strength characteristics of the target, penetration by the liner 8 or fragments 12 may heavily damage or destroy the target. However, if the target is an armored vehicle or other heavily armored target, the liner 8 or fragments 12 may not cause the desired degree of damage. To improve the destructive capability of the warhead, the liner 8 or fragments 12 may be provided with the ability to produce secondary reactions that cause additional damage. These secondary reactions commonly include incendiary reactions. As disclosed in U.S. Pat. No. 4,807,795 to LaRocca et al., pyrophoric metals are added to the liner to provide the desired incendiary effects. In LaRocca et al., a double-layered liner is disclosed, where a layer of dense metal provides the penetration ability and a layer of light metal, such as aluminum or magnesium, produces the incendiary effects.
While metals have been commonly used in liners, reactive materials have also been used. As known in the art and used herein, the term “reactive material” refers to a material comprising a metal that reacts with an oxidizing agent. Upon impact with a target, the reactive material of the liner produces a high burst of energy. A known reactive material includes an aluminum and polytetrafluoroethylene (“PTFE”) material, referred to herein as an “Al/PTFE” reactive material. PTFE is available from DuPont under the tradename TEFLON®. PTFE has the highest fluorine content of all fluoropolymers, is the most resistant fluoropolymer to chemical attack, and requires high processing temperatures to achieve its maximum strength. PTFE is used in reactive materials because its high fluorine content makes it a strong oxidizing agent. The Al/PTFE reactive material has good penetration ability in light armor or thin-skinned targets, such as aircraft, due to the density of the aluminum. The Al/PTFE reactive material also provides incendiary reactions because the reactive material ignites upon penetration into the target.
To form Al/PTFE high strength components, such as reactive fragments 12 for fragmentary warheads 10, the reactive material is pressed into billets or pressed preforms. The pressed preforms are then sintered and annealed at high temperatures, typically 350-390° C. Due to PTFE's high melting temperature of 342° C., these high sintering temperatures are necessary to form reactive materials using PTFE. The currently preferred technique for forming Al/PTFE fragments comprises blending the PTFE and aluminum in a solvent. The solution of Al/PTFE is spread on a tray and dried in an oven. The dried composition is then conditioned to 185° F. and pressed in a 185° F. heated die. The pressed preform is then heated to 350-390° C. for sintering. Since the PTFE is highly viscous at this temperature range, it maintains its approximate shape. The sintered preform is then cooled at a set rate to minimize cracking and maximize the mechanical properties of the Al/PTFE reactive material. The mechanical properties of the Al/PTFE reactive material are inversely related to the degree of crystallinity in the PTFE. In general, high crystallinity in the PTFE results in low tensile strength and high elongation. The current processing techniques available to form high strength components from Al/PTFE are limited due to PTFE's high viscosity at the 350-390° C. temperatures required for sintering.
To further increase the penetration ability of warheads, reactive materials comprising PTFE and metals with a higher density than aluminum have been produced. These higher density metals included tantalum and tungsten, which are more chemically reactive with PTFE at the sinter temperatures than aluminum. These Ta/PTFE and W/PTFE reactive materials were processed, using the same conditions as the Al/PTFE reactive material, to form 3.5-inch diameter and 1-inch diameter pucks. However, under these reaction conditions, the Ta/PTFE and W/PTFE reactive materials exhibited undesirable grain cracking resulting from volatile chemical reactions during the sintering process. The tungsten and tantalum reacted with trace amounts of hydrofluoric acid (“HF”) present at the temperatures used during the sintering process to produce highly volatile reaction products. The Ta/PTFE reactive material formed volatile tantalum fluoride compounds that were extremely exothermic. Accelerated Rate Calorimetry (“ARC”) testing of the Ta/PTFE material revealed an exotherm that occurred at only a few degrees higher than the sintering temperature. This exotherm occurred at 375° C. In addition, the strong exothermic reaction caused the Ta/PTFE reactive material to autoignite at 307° C. during an experimental sinter cycle. The W/PTFE reactive material off-gassed during the sintering process due to the formation of highly volatile tungsten fluoride compounds (such as WF6 and WOF4) that caused severe cracking of the pressed preforms. These highly exothermic reactions raised concerns regarding the safety of processing the Ta/PTFE reactive materials at the high temperatures necessary to process PTFE. The highly exothermic reactions also raised concerns regarding the quality of the W/PTFE reactive materials due to the observed cracking.
Reactive materials comprising a metal and a fluoropolymer have also been used in military pyrotechnics. In U.S. Pat. No. 5,886,293 to Nauflett et al., a process of producing energetic materials for use in military pyrotechnics is disclosed. The energetic material comprises a magnesium fluoropolymer, specifically magnesium/TEFLON®/Viton® (“MTV”). Viton® is a copolymer of vinylidenefluoride-hexafluoropropylene. The resulting energetic material is used to produce rocket motor igniters and aircraft decoy flares.
In light of the processing and safety problems associated with Ta/PTFE and W/PTFE reactive materials, it would be highly desirable to develop a reactive material having a high penetration ability that can be safely processed at temperatures lower than the 350-390° C. temperatures required to process PTFE. Ideally, the desired reactive material would be processed at temperatures below the autoignition temperature at which volatile metal fluoride compounds form.